Individuals who lose all or part of a leg have a residual limb to which a prosthetic foot is often attached through an elongated pylon. The attachment between the lower end of the pylon and the prosthetic foot approximates an ankle joint. However, in the past, pylons have been rigidly attached to prosthetic feet, thus creating a rigid ankle joint. Rigid ankle joints have typically relied on a cushion in the heel of the prosthetic foot to allow relative axial motion between the residual limb and the ground. However, this approach has proven to be inadequate because it makes the individual walk awkwardly, and prone to stumble when standing on an incline.
The basic problem with a rigid ankle joint is that it does not mimic a real ankle. As a result, prosthetic designers have developed pivotal ankle joints. Such ankle joints typically provide some motion in three orthogonal planes, namely the sagital, coronal, and transverse planes. A transverse plane is orthogonal to the longitudinal axis of the residual limb, and movement in the transverse plane is known as transverse adduction or abduction of the foot, or transverse rotation. A sagital plane is a vertical front-to-back plane, and movement in the sagital plane is known as either dorsiflexion in which the toe pivots upwardly or plantar flexion in which the toe pivots downwardly. The coronal plane is a vertical plane orthogonal to the transverse, and rotation in the coronal plane is coronal rotation, i.e., inversion or eversion of the foot.
Some of these pivotal ankles allow for these motions by attaching the residual limb to the prosthetic foot with a resilient material. The resilient material allows the residual limb to move relative to the prosthetic foot in any direction. One problem with such ankles is that they do not allow the resistance of the ankle to dorsiflexion and plantar flexion to be independently adjusted. It is known that it is desirable for the ankle to have greater resistance to dorsiflexion than to plantar flexion so the individual using it will have a natural walking gait. Thus, these ankles are inadequate because they make the individual walk awkwardly.
Some other pivotal ankles do allow independent adjustment of the resistance to plantar flexion and dorsiflexion. An example of such an ankle is U.S. Pat. No. 4,645,508, to Shorter et al. This ankle has a residual limb mounted vertically on a ball and socket joint having an outer sleeve which skirts the front and both sides of the shank of the ball. A ring of resilient material surrounds the shank of the ball and is fitted underneath the sleeve. During dorsiflexion the sleeve restricts expansion of the resilient material as it is compressed by the socket, thus providing resistance to dorsiflexion. During plantar flexion the resilient material is free to expand while it is compressed by the socket, thus providing less resistance to plantar flexion. One problem with this ankle is that because it uses the same ring for dorsiflexion and plantar flexion, it can only provide gross adjustments in resistance. For the individual to have a natural walking gait, fine adjustments are necessary.
Other pivotal ankles can provide finer adjustments by using different resilient materials for dorsiflexion and plantar flexion. An example of such an ankle is U.S. Pat. No. 3,851,337, to Prahl. The ankle disclosed in the Prahl patent has a shaft extending along the longitudinal axis of a residual limb which terminates in an eye socket. The eye socket is pressed onto a spherical bearing which is fitted on an axle mounted in a prosthetic foot. A second spherical bearing is fitted about the shaft of the residual limb, and a second eye socket is pressed onto the second bearing and is connected to a shaft extending toward the toe of the prosthetic foot. The shaft fits through a third spherical bearing and extends into a cylinder. Inside the cylinder both dorsiflexion and plantar flexion are independently resisted by separate cushions of resilient material. By adjusting the resilience of these two cushions, resistance to plantar flexion and dorsiflexion can be independently controlled. However, this ankle requires a complex linkage of sockets and bearings in order to do this while keeping the residual limb mounted vertically over the joint.
Therefore, there is a need in the art for a prosthetic ankle joint of simple construction which provides greater resistance to dorsiflexion than to plantar flexion.
One object of this invention is to allow an individual who has a residual limb to have a stable and natural walking gait.
Another object is to provide a prosthetic ankle joint which, like a natural ankle, allows dorsiflexion, plantar flexion, coronal rotation, and transverse rotation.
A further object is to provide a prosthetic ankle joint of simple construction.
Still another object is to provide a prosthetic ankle joint which is reliable.
A still further object is to provide a prosthetic ankle joint with greater resistance to dorsiflexion than to plantar flexion.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.